Argon2pure

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- Nothing changed yet.

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- Support ARGON2ID.
- Add ARGON2_TYPES constant.
- Python 3.6 support.

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- Close worker pool explicitly. For CPython this makes no difference, but
for a Python implementation without reference counting, like PyPy, this
will drastically decrease memory usage. (Eli Collins)

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- Add use_threads flag to use threads instead of processes. (Eli Collins)

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- Add ARGON2_VERSIONS constant. (Eli Collins)

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- Support and switch the default to Argon2 v1.3